Introduction to Biochemical Macromolecules and Thermodynamics in Microbiology

Chapter 1: Overview of Biochemical Macromolecules and Thermodynamics

General Structure of Biological Macromolecules

Biological macromolecules are large, complex molecules essential for life. They are typically polymers, meaning they are composed of repeating subunits called monomers. The directionality of these polymers is crucial for their function.

• Polymers: Large molecules made from smaller repeating units (monomers).

• Directionality: Refers to the orientation of the polymer chain, such as the N-terminus to C-terminus in proteins or 5' to 3' in nucleic acids.

• Examples of Macromolecules:

  • Proteins (polymers of amino acids)

  • Nucleic acids (polymers of nucleotides)

  • Polysaccharides (polymers of monosaccharides)

  • Lipids (not true polymers, but formed from glycerol and fatty acids)

• Monomers: The building blocks of polymers (e.g., amino acids, nucleotides, monosaccharides).

Example: A protein is a polymer made by linking amino acids in a specific sequence, giving it a unique structure and function.

Abundance of Important Elements in Organisms

Living organisms are composed of a limited set of elements, with carbon being the reference point for abundance. The table below compares the abundance of key elements in organisms and the universe.

Element	Abundance in Organisms (per 1,000 C atoms)	Abundance in Universe (per 1,000 C atoms)
Hydrogen	80–250	10,000,000
Nitrogen	1,000	1,000
Oxygen	60–300	1,600
Sodium	10–20	12
Magnesium	2–8	200
Phosphorus	8–50	3
Sulfur	6–40	80
Calcium	25–50	10
Iron	0.25–0.8	100
Copper	0.1–0.4	0.1

Purpose: This table highlights the relative abundance of biologically important elements, emphasizing the unique chemical composition of living matter compared to the universe.

Primary Structure and Directionality of Macromolecules

The primary structure of a macromolecule refers to the specific sequence of its monomer units. This sequence determines the molecule's properties and function.

• Proteins: Sequence of amino acids from the amino (N) terminus to the carboxyl (C) terminus.

• Nucleic acids: Sequence of nucleotides from the 5' phosphate end to the 3' hydroxyl end.

• Directionality: Each polymer has chemically distinct ends, which is critical for biological processes such as replication and translation.

Example: In DNA, the 5' end has a free phosphate group, while the 3' end has a free hydroxyl group.

Functional Groups in Biochemistry

Functional groups are specific groups of atoms within molecules that have characteristic properties and reactivity. They are essential for the structure and function of biomolecules.

• Definition: A functional group is an atom or group of atoms that imparts specific chemical and physical properties to a molecule.

Class of Compound	General Structure	Functional Group	Name
Alcohols	R–OH	–OH	Hydroxyl group
Amines	RNH2, R2NH, R3N	–NH2	Amino group
Carboxylic acids	R–COOH	–COOH	Carboxyl group
Phosphoric acids	R–O–PO3H2	–PO4	Phosphoric ester group
Thiol	R–SH	–SH	Sulfhydryl group
Additional info:	Other functional groups include ethers, esters, amides, and ketones, each contributing unique chemical properties to biomolecules.

Structural and Functional Biomolecules

Biomolecules can serve structural roles (e.g., in cellular membranes) or functional roles (e.g., as enzymes). Some molecules, such as ribonucleoproteins, combine both structural and catalytic functions.

• Structural biomolecules: Provide support and shape to cells and organelles (e.g., cytoskeleton, membranes).

• Functional biomolecules: Catalyze biochemical reactions (e.g., enzymes, ribozymes).

• Hybrid structures: Some complexes, like ribosomes, contain both protein and nucleic acid components.

Example: Enzymes are proteins that accelerate chemical reactions, while ribozymes are RNA molecules with catalytic activity.

Thermodynamics in Biochemistry

Thermodynamics governs the energy changes in biochemical reactions. Understanding these principles is essential for predicting whether a reaction will occur spontaneously.

• Free Energy (G): The energy available to do work in a system.

• Enthalpy (H): The heat content of a system at constant pressure.

• Entropy (S): A measure of molecular disorder or randomness.

• Free Energy Change (ΔG): Determines the spontaneity of a reaction.

Key Equation:

$\Delta G = \Delta H - T\Delta S$

• ΔG < 0: Spontaneous (exergonic) reaction; energy is released.

• ΔG = 0: Reaction is at equilibrium.

• ΔG > 0: Nonspontaneous (endergonic) reaction; energy is required.

• ΔH < 0: Exothermic; heat flows from the system to the surroundings.

• ΔH > 0: Endothermic; heat flows into the system from the surroundings.

• ΔS > 0: System becomes more disordered.

• ΔS < 0: System becomes more ordered.

Example: The hydrolysis of ATP to ADP and phosphate is a spontaneous reaction with $\Delta G^\circ' = -30.5$ kJ/mol.

Types of Thermodynamic Systems

Thermodynamic systems are classified based on their ability to exchange energy and matter with their surroundings.

• Open system: Exchanges both energy and matter (e.g., living cells).

• Closed system: Exchanges only energy, not matter.

• Isolated system: Exchanges neither energy nor matter.

Example: A cell is an open system because it exchanges nutrients and energy with its environment.

Additional info:

• Spontaneity does not imply reaction speed; some spontaneous reactions may occur very slowly without a catalyst.

• Enzymes lower the activation energy of biochemical reactions, increasing their rate without altering ΔG.